Aquatic Animals

hydrobionts, animals whose entire life is spent in water. Because water is 800 times denser than air on the average, animals can exist in it whose transparent, jelly-like bodies lack a solid integument or supporting skeletal apparatus (such as jellyfish, siphonophores, ctenophores, and salpas). The density of water is also responsible for the methods of locomotion by cilia or flagella characteristic of many aquatic animals (such as most protozoans, certain worms, and coelenterates, as well as larval forms of sponges, coelenterates, worms, mollusks, echinoderms, and others). The great density of water enables very tiny aquatic animals (plankton), which are capable only of weak active movements, to maintain themselves in water by using simple adaptations in the form of minute air bubbles, or fat droplets in their body, or long, thin processes that increase the body surface. Only among aquatic animals do immobile, sessile forms occur; such forms can exist because of the movement of water, which constantly brings in food ir the form of living and dead planktonic organisms and disperses fertilized eggs and larvae, thus ensuring the distribution of sessile forms.

In the great majority of aquatic animals (invertebrates and fish), fertilization is external, with the ejected eggs and spermatozoa meeting because of the motion of the water. Reproduction by division and budding is peculiar to aquatic animals alone. Respiration takes place through special external processes—gills—or the entire body surface.

Paleontological data, which show that the remains of animals in the most ancient layers of the earth’s crust consist of marine forms, and the comparative anatomical and embryological data are evidence of the fact that life on earth began and developed in water. However, the progressive development of animals in water did not go beyond the class of fish. The absence of higher groups of vertebrates among the primary aquatic animals may be due largely to the fact that water, which on the average contains dissolved oxygen equivalent to one-thirtieth of the amount found in air, cannot satisfy an organism’s strongly growing need of oxygen under conditions of the intensified metabolism characteristic of the higher classes of the animal world.

Some forms of higher terrestrial classes of animals, which descended from aquatic ancestors, returned in the course of evolution to the aquatic mode of life. These secondary aquatic animals include pinnipeds, cetaceans, and sirenians among the mammals; some turtles and snakes among the reptiles; some beetles and true bugs among the insects; and some freshwater and land snails among the mollusks. Despite the high morphological and physiological adaptability of secondary aquatic animals to life in water, they continued to breathe air.

Aquatic animals are divided into two main groups: marine and freshwater. Paleontological and physiological data show that modern freshwater fauna originated from marine forms. The original forms of vertebrates and insects lived in bodies of fresh water and after emerging onto land started the evolution of terrestrial fauna.